Circuits for the production of pure modulated carrier waves



Dec. 15, 1925- ,565,3 7

R. E. CORAM CIRCUITS FOR THE PRODUCTION OF PURE IODULATED CARRIER WAVESFiled Sept. 18. 1923 Patented Dec. 15, 1925.

UNITED STATE s PATENT OFFICE.

ROY E. CORAM, 0F NEWARK, NEW JERSEY, AB SIGNOB TO WESTERN ELECTRIC CO!-IPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION 01' NEW YORKCIRCUITS FOR THE PRODUCTION 0! Application illed September 2 '0 all whomit may concern Be it known that I, Roy E. Comm, a citizen of the UnitedStates, residing at Newark, in the county of Essex, State of New Jersey,have invented certain new and useful Improvements in Circuits for theProduction of Pure Modulated Carrier Waves, of which the following is afull, clear, concise, and exact description.

This invention relates to modulating circuits and more particularly tocircuits for the production of pure modulated carrier waves.

The ordinary circuit for modulatin a carrier wave in accordance with aSigma or other control wave produces a complex resultant wave comprisinga pure modulated wave and an unmodulated component of the carrier wavefrequency. It is frequently desirable to produce a modulated "carrierwave having no unmodulated carrier frequency component. An. example of acircuit for accomplishing this purpose is the balanced modulator of J.R. Carson Patent No. 1,449,882, March 27, 1923. It is an object of thepresent invention to provide a circuit for producing pure modulatedwaves with the simplest possible apparatus.

According to the present invention, signal -or other modulating wavesare impressed upon the input circuit of an electron discharge device ofa well-known three-element type, preferably highly evacuated. A sourceof carrier waves is connected to the discharge electrodes and to a pathincluding an equivalent impedance in such manner as to supplyalternating currents in par-' allel to the variable impedance betweenthe discharge electrodes and to a constant impedance. A signaltransmission circuit is differentially associated with the alternatingcurrent paths including the discharge electrodes and the fixed impedanceelement and accordingly receives no current as long as the impedancebalance is maintained. When,

however, the impedance of the discharge device is varied by incomingsignals, the

balance is disturbed and carrier waves'corresponding to the resultingdifierence currents in the two alternating current paths,

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as to the details of its organization and its mode of operation will bebest understood by reference to the following description taken inconnection with the accompanyin drawing in which Fig. 1 illustrates are. io telephone system embodyin the invention, and Fig. 2 illustrates amodi cation of the modulator circuit of Fig. 1.

Referring to Fig. 1 a source of speech or other control currentsrepresented by the microphone 1, in series with a battery 2, isassociated by means of the transformer 3 with the input circuit of a,three-element electric discharge device 4 of well-known type. Connectedin series in the input circult of the grid or impedance control element5 and the cathode 6, are the secondary of transformer 3, a choke coil 7'and a polarizing battery, 8 for maintaining the norimpedance 15. Each ofthese circuits includes a large blocking capacity element 16.

The .two alternating current circuits or branches are connected by meansof threewinding transformers 17 to a transmission circuit 18, in suchmanner as to induce opposing electromotive forces therein. The circuit18, which is coupled to a transmission circuit 19 by transformer 20,includes a band filter 21 which passes with low and substantiallyuniform attenuation, fre uencies corresponding to one side band 0 themodulated carrier wave and substantially extinguishes currents outsidethis frequency range.

In an actual operation of this circuit, the device 4 consisted of astandard tele hone repeater tube. This tube was operate with a negativegrid potential impressed by means of source 8, so that its alternatnigcurrent impedance between the cathode and anode was of approximately6000 ohms. The condenser 16 in the alternating current path between thecathode and anode had a capac- LOO ity of one microfarad, and thecondenser 16 in the other branch was of like ca acity.. It

- was found in operation of this evice, that without the inductancedevice 7, there was atendency for an alternating electromotive force ofthe carrier frequency to be induced in the grid circuit leads, and to beimpressed upon the input elements of the discharge device 4, thus givingrise of an amplified carrier frequency current in the alternatingcurrent plate circuit path, in addition to that directly impressed onthis path by source 12. To prevent this effect, the element 7 havinginductance of approximately 10 henrys, was inserted in t 1e gridcircuit. The. potentiometer 13 enables the alternating electromotiveforce supplied to the discharge device to be made such that the deviceis not overloaded, thus making it ssible to eliminate, to a largedegree, a istortion giving rise to resulting undesired frequencycomponents which will not be neutralized by the current through fixedimpedance.

The band filter 21 may be designed in accordance with well-knownprinciples, as, outlined in United States patent to G. A.

-Cam bell,.No. 1,227,113, May 22,1917. The

circult 19, although illustrated as an antenna, may obviously be atransmission conductor of any sort, and the transformer 20 may bereplaced by any suitable coupling.

In operation, the alternating currents induced in the two pathstraversing space discharge device 4 and the impedance element 15 arenormally equal and in phase. In consequence of the differentialconnection of the transmission circuit 18, the electromotive forcesinduced therein are oppositely directed and neutralized. When speech orother signal control currents are impressed by means of the transformer3 upon the input circuit of device 4, thus varying its impedance, thecurrent in this branch alternately increases and diminishes,

thus destroying the balanced condition with respect to circuit 18. Therewill accordingly be impressed in circuit 18, a carrier wave of thefrequency of source 12, the amplitude of which is'proportionate to thevariation currents impressed upon the input circuit of device 4. Themodulated carrier .wave supplied in the transmission circuit 18 comrises, asis well known, an upper side ban and a lower side band, equallyremoved in frequency, from the frequency of the carrier source 12. Inorder to improve the quality of signals received at the receivingstationv and to economize in the frequency range of the wavestransmitted,

the filter 21 is provided to transmit one of the side bands and toeliminate waves of other frequencies including the other side hand. Toan extent, this fi ter 21 may serve to suppress any unmodulatedcomponent of the carrier frequency, should the impedance 15 fail tomatch the normal impedance of device 4. However, the percentagefrequency difierence between the carrier frequency and the contiguousfrequenciesfof the side band to be transmitted, is, in general, small,and it is therefore very desirable to make the balance as accurate aspossible. For this purpose, the resistance 15 may preferably bevariable.

Fig. 2'shows a similar arrangement with elements similarly designated. Acoupling circuit between the signal source and the inputcircuit of'tube4 includes a potentiometer 31 to regulate the magnitude of themodulating current impressed thereon. To generate cagrier current, thereis provided a vacuum tube oscillator comprising a space discharge device32 inthe plate filament circuit of which there is-included a closedoscillatory circuit 33, tuned'to the desired carrier wave frequency andcoupled by transformer 34 in feed-back relation-to its input circuit.vThe cathode of oscillator 32, preferably heated from the source 35, isin series with the cathode of modulating device 4. Space current issupplied to the discharge devices 4 and 32 from the common source 9,througllli the choke coils 10 and36 respectively, w 'chv preventby-passing of carrier. frequency current to thefspace current source 9.A resistance 37 in the alternating current plate circuit path of theoscillator is provided to stabilize the oscillations of the oscillatorand prevent overloading. A potentiometer resistance 38 is connectedbetween the cathode of discharge device 4 and the midpoint of thewindings of transformer 17 in the same relative position as thesecondary winding of transformer 14 of Fig. 1. The oscillator 32impresses an alternating electromotive force between the common cathodeconnection and the contact of a variable tap 39 on the potentiometerresistance 38. A blocking condenser 40 of large capacityin series withthe tap 39 prevents transmission of direct current through theresistance element 38. This element is preferably of large impedance as,for example, 40,000 ohms. The operation of this "connected thereto, onepath 0 discharge impedance element, a constant impedance ofsubstantially equivalent magnitude, and two circuits differentiallyconnecting said source to said work circuit and including respectivelysaid variable electric discharge impedance and said constant impedance,whereby alternatin current is supplied to said work circuit on y duringand in proportion to variations in said impedance.

2. A modulating system comprising a source of carricr waves, a twoathcircuit which includes a variable thermionic impedance, and the secondpath of which includes a constant impedance simulating in magnitude thenor-' ma impedance of said first-mentioned path, and means for derivingfrom said circuit a current proportionate to the difference between thecurrents in the two paths.

3. In combination, a source of alternating current, a three-elementelectric discharge device having discharge electrodes and an impedancecontrol element, a metallic circuit having an impedance comparable inmagnitude to that between said discharge electrodes, means for imressing an alternating electromotive force ii'om said source u u saiddischarge electrodes and a like e ectromotive force upon said circuit, awork circuit, and means for impressing upon said work circuit anelectromotive force corresponding to the difference between thealternating current in said impedance circuit and that'between saiddischarge electrodes. k

4. In combination, an electric discharge device having dischargeelectrodes and means for controlling the im ance therebetween, a closedmetallic pat of predetermined impedance, a source of alternating currentassociated with said discharge device and said path to sup 1y currentthereto in parallel, and a wor circuit'difierentially connected to saiddevice and saidpath whereby difierences of current traversing saiddevice and said path produce a resultant current in said work circuit.

, 5. A system comprising an electric discharge device having dischargeelectrodes and means for controlling the impedance therebetween, asource of space current for said device, a two-branch circuit includingsaid discharge device in one branch and a constant impedance equivalentto the normal impedance of said device in the other branch, a commonsource for supplying alternating current to both of said branches, and awork circuit to which said branches are connected in' opposition wherebycurrent is supplied to said work. circuit only when the impedancecontrolling means changes the impedance of said device from its normalvalue. a

6. A signal system comprising a source of signaling currents, a sourceof carrier currents, an electric discharge device having an anode, acathode, andan impedance control element, an input circuit associatedwith said source of signaling current and connected to said cathodeandi'mpedance control element, ,a second circuit connected with saidcathode and-anode and associated with ROY E. CORAM.

